This invention relates to overthrust protection for gas turbine power plant, primarily although not necessarily exclusively for aero gas turbine engines.
Gas turbine power plant are commonly operated under the control of an electronic engine control system, one major element of which is a fuel control system to control the supply of fuel to the combustor of the gas turbine. The fuel control system calculates the gas turbine""s fuel demand based on the desired level of thrust, indicated for instance by a throttle lever position set by the operator (e.g. the pilot or autopilot of an aircraft in the case of an aero engine), accounting also for other operating parameters of the gas turbine such as pressures and shaft speeds. This calculated fuel demand is then applied by the control system to set a fuel metering valve which controls the flow rate of fuel to the combustor.
It is also important that the control system includes safeguards to protect the gas turbine against faults occurring, for example, in the control system itself, the sensors from which its inputs are derived, or within other components of the gas turbine. One such safeguard, with which the present invention is particularly concerned, is the prevention of very high, uncontrolled thrust levels (xe2x80x9coverthrustxe2x80x9d) arising as a result of faults within the gas turbine or its control system. This safeguard is particularly important in the context of aero gas turbine engines, where such uncontrolled high thrust levels can give rise to undesirable, and potentially dangerous imbalances in the thrust developed by multiple engines of an aircraft.
One known measure to provide some protection against overthrust is the use of what is termed an xe2x80x9coverspeed protection unitxe2x80x9d or xe2x80x9cOPUxe2x80x9d as part of the fuel control system. This is a device that has as its inputs the shaft speeds for e.g. the high pressure and low pressure shafts of a multi-stage gas turbine. The output from the device controls a fuel shut-off valve which is operable to cut-off the fuel supply to the combustor, thus shutting down the engine, when the shaft speeds reach excessively high levels, indicative of an overthrust condition.
Although the use of an OPU goes some way to providing overthrust protection it is by no means a complete solution and it is recognised that some enhancement is required to enable the aircraft to be fully protected against overthrust.
The present invention is generally concerned with providing a system and method offering an improved safeguard against overthrust.
In one of its aspects there is provided an overthrust protection system for a gas turbine in which fuel is supplied at a controlled flow rate to a combustor of the gas turbine, the overthrust protection system comprising:
a bypass valve for diverting a controlled amount of fuel flow away from the combustor;
detection means for detecting an overthrust condition of the gas turbine; and
overthrust control means for controlling the bypass. valve to divert the controlled amount of fuel flow in response to a detected overthrust condition.
In another aspect, the invention provides a method of verthrust protection for a gas turbine in which fuel is supplied at a controlled flow rate to a combustor of the gas turbine, the method comprising:
detecting an overthrust condition of the gas turbine; and
diverting a controlled amount of fuel flow away from the combustor in response to the detected overthrust condition.
Diverting some of the fuel flow in this manner brings about a reduction in the thrust, allowing the overthrust condition to be controlled without resorting to a complete shut down of the gas turbine. This approach can offer particular advantages when shutting down the gas turbine might itself have undesirable effects, potentially more threatening than the overthrust condition itself, for example in the case when an overthrust condition occurs during the final approach of an aircraft when landing.
The bypass valve may be an open/closed device to divert a fixed fraction of the fuel flow when the overthrust condition is detected, however it is more preferable to use a valve which can be controlled to divert a variable fraction of the flow. In this way the reaction of the overthrust protection system can be more readily controlled to suit the circumstances of any particular fault condition.
In order to determine the presence of an overthrust condition, the speed of one or more shafts of the gas turbine can be sensed and compared with a predetermined threshold, much in the same way as the shaft speeds are detected by the known OPU. Preferably, however, these measurements are supplemented by further parameters indicative of the operating state of the gas turbine or the overall system (e.g. aircraft) of which it forms a part, in order that the overthrust condition can be determined with reference to these operating states. Such further parameters can usefully include, for instance, throttle position (i.e. demanded thrust), xe2x80x98weight on wheelsxe2x80x99 indicator and static air pressure.
Advantageously, the ability to shut down the gas turbine, for example by employing a fuel shut-off valve in addition to the bypass valve, is retained, because there may be situations in which a complete shut down is preferable to a reduction in thrust. In such situations, the overthrust control means preferably controls the operation of the fuel shut-off valve, closing this valve whilst the bypass valve also remains closed.
The normal supply of fuel to the combustor, that is the flow of which a fraction can be diverted by operation of the bypass valve, can be controlled in a conventional manner independently of operation of the overthrust protection system. By keeping the two systems independent, the overthrust protection system can appropriately react to overthrust conditions even when brought about by faults in the fuel control system itself.
This independence of the fuel and overthrust systems preferably extends also to the sensors from which their inputs are derived. For example, where both the fuel control system and overthrust protection system require shaft speeds as inputs, separate shaft speed sensors are desirably provided for each system. Likewise, when used, inputs to the overthrust protection system such as xe2x80x98weight on wheelsxe2x80x99 indication, throttle angle and static air pressure should preferably be independent from the normal fuel supply system.